Cardiac imaging can be used to determine local myocardial function, perfusion and viability. Nowadays, such images can be obtained using various modalities such as magnetic resonance imaging (MRI), computed tomography (CT), ultrasound (US) and nuclear medicine (PET/SPECT). The images for function, perfusion and viability may be obtained using one modality (MRI) or using hybrid imaging, for instance PET/CT.
After acquisition, the images are analyzed independently, resulting in a number of measurements performed at different positions at the myocardium. The functional images are used to quantify wall motion abnormalities, the perfusion images are used to quantify perfusion deficits and the viability images are used to quantify scarring of the myocardium.
These measurements produce quantitative data that are usually displayed in so-called bull's eye plots, in which measurement values are color coded and projected on a plane perpendicular to the long axis.
Current cardiac analysis software packages generate bull's eye plots in which each ring in the bull's eye plot corresponds to a particular slice in the stack of acquired images. If a comprehensive cardiac exam is performed, bull's eye plots are made based on the (local) measurements derived from each scan. These bull's eye plots are then included in a comprehensive report. Functional measurements that can be displayed in a bull's eye plot are: end diastolic wall thickness, end systolic wall thickness, wall thickening (both absolute and relative), wall motion, time of maximum contraction, maximum wall thickness etc. Perfusion related measurements that can be displayed in a bull's eye plot include time of peak intensity, time of maximum upslope, upslope ratio rest/stress, etc. Viability related measurements that can be displayed in a bull's eye plot are: percentage of viable tissue, thickness of viable tissue, a transmurality index, etc.
Unfortunately, current scanning procedures do not yet allow acquiring functional, perfusion and viability images of the same resolution. Consequently, the acquired image stacks differ with respect to the slice thickness, slice spacing and the number of slices. Therefore, the derived bull's eye plots consist of arbitrarily distributed different numbers of rings, such that it becomes difficult to compare the complementary information comprised in function, perfusion and viability bull's eye plots. This is illustrated in FIG. 1A showing bull's eye plots derived from a comprehensive cardiac MR exam comprising functional image data containing 10-15 contiguous slices of thickness 8 mm (a), from perfusion images of 3 slices of thickness 20 mm and gaps of 10 mm (b), and viability images of 15-20 slices of thickness 6 mm (c). The bull's eye plots in FIG. 1A are scaled such that they are of substantially the same size.
A standardized myocardial segmentation for tomographic imaging of the heart has been proposed by the American Heart Association (AHA). The AHA proposal is described in AHA Scientific Statement: Standardized Myocardial Segmentation and Nomenclature for Tomographic Imaging of the Heart, Circulation 2002; 105:539-542 (available online at http://circ.ahajournals.org/cgi/content/short/105/4/539). FIG. 1B schematically shows left ventricular bull's eye plot sectors (a) and the corresponding myocardial slices and sectors (b) according to the AHA model. AHA recommends dividing the left ventricle into equal thirds perpendicular to the long axis of the heart to generate three slices of the left ventricle: the circular basal slice 1 comprising sectors 1 to 6, the mid-cavity slice 2 comprising sectors 7 to 12, and the apical short axis slice 3 comprising sectors 12 to 16. The last sector, the apex 17, is shown in a vertical long axis slice 4. The slice thickness should be selected on the basis of modality-specific resolution and clinical relevance and should be less than 1 cm. While the AHA segmentation allows comparing different cardiac study quantitative analyses, the proposed visualization often ignores large chunks of data which are not comprised in said three slices.